It is now known that a rather precise anatomical relationship exists between spinal motor neurons and the muscle fiber with which they make synaptic contacts. Single motor units within a muscle are aggregated into neuromuscular compartments, each of which is innervated by a primary branch of the muscle nerve, and the motor neurons innervating a compartment are topographically mapped in the spinal cord. This spatial relationship between motor neurons and muscle fibers is termed motor unit specificity (76). The proposed study will investigate the roles played by two normal processes of postnatal growth, input elimination and histogenetic cell death, in the establishment of motor unit specificity. At birth all mammalian muscle fibers receive synaptic input from more than one motor neuron but shortly thereafter all but one of these inputs are eliminated. Similarly, up to 30% of spinal motor neurons are reported to die postnatally. Experiments are proposed, using the compartmentalized cat lateral gastrocnemius muscle as a model (21-26), to test hypotheses that at least one role played by both input elimination and histogenetic cell death is the establishment of an adult pattern of motor unit specificity from a less precise pre-natal spinal cord-to-muscle projection. It is anticipated that the results of this project will be significant to motor control and developmental neurobiologists in helping to elucidate factors which contribute to the establishment and maintenance of precise synaptic connections in the nervous system, especially as related to the mechanisms underlying disorders of movement.